PDFTK-Java not installing (DEBIAN 10)

[the-hotmann]

About the Formula: https://github.com/Homebrew/homebrew-core/blob/master/Formula/pdftk-java.rb

When I run:
brew install pdftk-java
like recommended here:
https://formulae.brew.sh/formula/pdftk-java
then it starts
brewing it. But fails with:

Error: An exception occurred within a child process:
  TypeError: no implicit conversion of nil into String

On the PAGE the statistics tell me there is not a single failed build Error so I'm curious about if I did something wrong?

My System:
Debian 10

user@main:~$brew -v
Homebrew 2.7.1
Homebrew/linuxbrew-core (git revision 263937; last commit 2020-12-30)

[gromgit]

Your Homebrew seems fairly up-to-date, but just to be sure there isn't anything odd about your installation, what's the output of brew config and brew doctor?